The present invention relates to a method of and apparatus for color-copying. More particularly this invention concerns a scanning method and apparatus for a color-copying system.
A color-copying method is known wherein the original is irradiated with a white light and several color components are derived from the irradiated portion of the original. Each of these color components is applied to a separate light-sensitive element and converted thereby into corresponding electrical signals.
When the scanning is done in a line-by-line fashion as described in the above-cited copending patent applications, each line or strip on the original is broken down into a multiplicity of spots or sections. Each color-component line is read by a device working in accordance with the principle of an iconoscope, a orthicon, or a vidicon. Such devices all use a scanning electron beam which is caused to pass over the line being scanned so as to produce a succession of electrical signals each of whose amplitude is proportional to the intensity of the respective color of light at the respective section. The mosaic arrangement in such a pickup or transducer device inherently separates the band or strip into a succession of sections.
The application of such an arrangement to color-copying, however, presents a considerable amount of difficulty. Since each color-component strip must be scanned by a respective light spot or electron beam is of frequently very low intensity, it is necessary to set the transducer up for maximum sensitivity. This has the frequent result of overloading it so that the device is ruined and the expensive pickup device must be replaced. Alternately it is possible to reduce scanning speed so that the electron beam or light spot rests a relatively long period of time on each section, but this has the concomitant disadvantage of greatly increasing copy time.
When the copy is produced electrophotographically the separate color components must be applied at different times to the copy sheet, with developing and fixing of each color component before application of the next one. Proper electronic masking of the color components is only possible when all of the color components are scanned at the same time. Thus, the scanning for each color component requires a three-channel scanner. The other color components are scribed on the copy sheet after the first component. When three-channel scanning is used, it is therefore necessary to store all of the color component signals. With the picture having dimensions of 90 by 120 mm with 10 lines/mm each color component requires 1.08 .times. 10.sup.6 places in the memory for the analog signals. For an analog/digital conversion it is necessary to take into account 128 gray levels. Thus, the memory must have a capacity of 1.08 .times. 10.sup.6 words each of 7 bits. At a scanning rate of three images per second the analog/digital and digital/analog conversion must operate with a scanning frequency of approximately 4 MHz.
For this reason it is frequently considered necessary to use a separate scanner for each scriber. This eliminates the need for an enormous memory capacity but brings up the problem of registration. The three or four scanning tubes must be controlled so that the individual color components register exactly with one another on the copy sheet. Even if the images are shifted by so much as a few line sections relative to each other, even as few as two or three per thousand, the copy can be unusable.